Our long-term goal is to establish the structural features responsible for the cardioprotective functions of HDL in humans, which may have important implications for predicting CVD risk and developing HDL-targeted therapeutics. We have recently shown that apolipoprotein A1 (APOA1), HDL?s major apolipoprotein, can assume three different antiparallel isomeric structures we term rotamers. Strong preliminary data suggests that the different rotamers exist in humans, and that they affect HDL size and ability to bind different proteins, and have different abilities to promote cholesterol efflux from macrophages by the ABCA1 pathway. To determine the physiological relevance of the APOA1 rotamers, we therefore propose two specific aims. First, we will engineer Apoa1-/- mice to express mutated forms of human APOA1 that cause HDL to selectively form specific rotamers of APOA1 in vivo. We will then determine the impact of the different rotamers on HDL cholesterol efflux capacity, HDL size, HDL protein cargo, and atherosclerosis in LDL receptor-deficient mouse models. Second, we will complement our animal studies with analyses of HDLs of humans with and without carotid atherosclerotic disease, and with and without diabetes. We will determine whether the distribution of specific rotamers, HDL subspecies, and HDL protein cargo associate with cholesterol efflux capacity and whether they associate with or predict atherosclerotic disease in these subjects. Because preliminary studies show that HDL of diabetic patients have impaired cholesterol efflux capacity, and diabetic patients are at greatly increased risk of cardiovascular disease, our proposed investigation of the structural features of HDL that associate with both impaired HDL function and atherosclerotic disease could provide important insights into HDL-associated factors that are cardioprotective but are independent of HDL cholesterol levels.